Social bonds are critical to normal human functioning and, in animals, are essential for social organization. However, very little is known regarding the biological basis of adult attachments. The purpose of the proposed research is to study the physiology of social attachment and selective aggression in the context of pair bonding and monogamy. An animal model has been developed, based on the natural tendencies of monogamous rodents to show selective social behaviors. The proposed studies will focus on the behavioral effects of the peptides, oxytocin and vasopressin, in the monogamous prairie vole (Microtus ochrogaster), and will make selected comparisons with less social, nonmonogamous montane voles (Microtus montanus). We have found that centrally administered oxytocin and vasopressin facilitate the development of partner preferences, and propose here to continue to use vole models to study the behavioral effects of these peptides. Oxytocin, especially, is widely manipulated by childbirth and lactational practices, and often is administered to facilitate labor and lactation. Vasopressin-like analogues are prescribed to prevent bed- wetting in children, and are the basis for drugs that are being developed to treat high blood pressure and memory deficits. The proposed research will continue to compare the role of oxytocin and vasopressin in selective social contact and aggression. Sex differences are seen in the time course of partner preferences and in the development of selective aggression. We also have recently discovered sex differences in the behavioral effects of stress (or related hormonal treatments); stress (in this case 3-min of swimming) or corticosterone treatments facilitate partner preference development in males and inhibit this behavior in females. We propose to continue to describe sex differences in these behaviors and in the physiological processes that may underlie these differences. We also will extend this work to examine the hypothesis that sex differences in the behavioral effects of oxytocin and vasopressin might explain the observed sex differences in the behavioral effects of stress. This research may provide a better understanding of fundamental sex differences in the neural substrates responsible for social attachment.